Optical interconnects based on ion-exchanged (IOX) waveguides realized in thin glass substrates are a promising alternative to electrical (e.g. copper), or optical-fiber-based high-density, high bit-rate, short distance (less than 1 m) links for high-performance computing and data center applications. Such IOX waveguides have the advantage of dense routing, flexibility, integration, and co-packaging with electronic integrated circuits.
A high bit-rate optical signal is typically delivered to optical interconnect waveguides through an optical fiber. Thus, a low-cost, low-loss connectivity solution from a single mode fiber to an IOX waveguide is desirable. A standard approach is to use an end-to-end coupling (also called edge coupling) between the single mode fiber and lox waveguide. Such coupling requires that the glass edge and fiber end be processed to achieve an optical quality (i.e., smoothly polished) surface for low-loss coupling. This coupling also requires that mode sizes (i.e., mode-field diameters) of the guided mode of the fiber and the guided mode of the IOX waveguide be closely matched.
An alternative approach to end-to-end coupling is to use evanescent coupling between the fiber and the IOX waveguide. Unfortunately, efficient evanescent optical coupling between a fiber and an IOX waveguide requires that the separation as well as the alignment between the fiber and the waveguide be controlled to challenging tolerances, e.g., to micron or even sub-micron levels. While evanescent coupling does not require matching of the fiber and waveguide modes shapes, it does require matching propagation constants of the fiber and waveguide guided waves. In evanescent coupling, the optical power transfer mechanism occurs all along an interface that is typically parallel to the direction of the travel of the guided mode, as opposed to end-to-end coupling where the power transfer occurs abruptly at an interface perpendicular to the direction of travel of the guided mode.
While evanescent coupling could provide some advantages over end-to-end coupling for an optical coupler, shortcomings in the consistency of the IOX fabrication process can result in significant variations in the properties of the IOX waveguides. In addition, the tight alignment and spacing requirements between the fiber and the IOX waveguide as well as the inherent mismatch in propagation constants have presented significant obstacles in achieving an efficient evanescent optical coupler using a single mode fiber in combination with an IOX waveguide formed in glass.